The present invention is generally related to control of electromechanical machines, and, more particularly, the present invention is related to method and system for controlling a permanent magnet (PM) machine without using high resolution absolute rotor position information.
In the starting and control of permanent magnet machines using sinusoidal current control, the absolute position of the rotor of the machine is generally required to appropriately position the stator""s current vector relative to the rotor""s magnet flux vector in order for the machine to develop a desired level of torque.
It is known that various techniques have been proposed to determine absolute rotor position. Some of the proposed techniques require the use of resolver devices. Although resolver devices can accurately determine the absolute position of the rotor, such devices generally require tight positional tolerances and associated driving circuitry, and thus their relatively high cost incrementally adds to the overall cost of a propulsion system that uses permanent magnet machines.
Other techniques have attempted to determine the absolute position of the rotor without the use of sensors. Unfortunately, sensorless techniques may require computationally intensive algorithms and may not be suitable to applications, such as those using flywheel alternator starter systems, where a relatively high initial torque is desired so that, for example, an internal combustion engine coupled to the starter system can be started as quickly as possible under extreme environmental conditions.
Unfortunately, absolute position sensors based on standard position-pulse techniques have limitations since such sensors may not be effectively utilized for applications that require high initial starting torque since the absolute position is not determined until a pulse indicative of relative position of the rotor is received by a controller. For example, the rotor position determination may not occur for up to about one mechanical revolution of the rotor, during which the controller would be unable to appropriately position the current vector relative to the flux vector. This condition could lead either to reduced torque generation or even to negative torque generation.
In view of the foregoing issues, it is desirable to provide techniques capable of producing high initial starting torque without having to use a high resolution absolute position sensor. It would be further desirable to use a low-cost and reliable sensing scheme that allows a standard vector controller that normally operates in a sinusoidal alternating current (AC) mode of operation to run during start up of the machine in a brushless direct current (BLDC) mode of operation to take advantage of the relatively higher torque characteristics that are achievable during the BLDC mode of operation. It is also desirable to be able to seamlessly transition or crossover from the BLDC mode of operation to the sinusoidal mode of operation once the startup of the machine is achieved. As will be readily understood by those skilled in the art, the use of the expression brushless direct current mode of operation is a bit of a misnomer since the mode is not truly a DC mode, if by DC one means a mode whose machine voltages and currents are unidirectional for a given condition of speed and torque. In fact, the voltages and currents of the machine generally vary trapezoidally in the BLDC mode of operation, however, the expression BLDC as used herein is consistent with traditional and well-understood usage in the field of electrical motors. For readers desiring further background regarding the operation of brushless DC motors, see 4th Ed. of textbook titled xe2x80x9cElectric motorsxe2x80x9d by Cyril G. Veinott and Joseph E. Martin at pp. 261-263, published by McGraw-Hill Book Company.
U.S. patent application Ser. No. 09/932,197, assigned to the same assignee of the present invention and herein incorporated by reference, discloses innovative techniques for controlling a permanent magnet machine using a sensor assembly for sensing rotor sector position relative to a plurality of angular sectors and an incremental sensor for sensing angular increments in rotor position. It would be desirable to further improve the control techniques disclosed in the above-identified patent application so that one can achieve the benefits of vector control operation, upon transitioning from the BLDC mode of operation, without having to use any incremental sensor. That is, it would be desirable to provide low-cost and reliable techniques capable of producing high initial starting torque during the BLDC mode of operation, and further capable of accurate vector control, without employing any incremental sensor, upon the machine reaching a predefined mode-crossover criterion, such as reaching a predefined mode-crossover rotor speed, or machine voltage limit corresponding to the predefined mode-crossover criterion.
Generally, the present invention fulfills the foregoing needs by providing in one aspect thereof a method for controlling a permanent magnet machine. The method provides a sensor assembly for sensing rotor sector position relative to a plurality of angular sectors. The method allows starting the machine in a brushless direct current mode of operation using a calculated initial rotor position based on angular sector position information from the sensor assembly. Upon reaching a predefined mode-crossover criterion, the method allows switching to a sinusoidal mode of operation using rotor angle position based on extrapolating angular sector position information from the sensor assembly.
The present invention further fulfills the foregoing needs by providing in another aspect thereof, a system for controlling a permanent magnet machine. The system includes a sensor assembly for sensing rotor sector position relative to a plurality of angular sectors. A rotor angle processor includes an angular sector assigner configured to assign rotor position based on angular sector position information from the sensor assembly, with an initial rotor position being used for starting the machine in a brushless direct current mode of operation. The rotor angle processor further includes an extrapolator configured to determine rotor angle position based on angular sector position information from the sensor assembly. A switching module is responsive to a switching signal from a mode-crossover controller to pass rotor position from the extrapolator. The rotor position information from the extrapolator is used to provide a sinusoidal mode of operation, in lieu of the brushless direct current mode of operation, upon the mode-crossover controller determining whether a predefined mode-crossover criterion has been reached.